The present invention relates to a positive-working photosensitive composition or, more particularly, to a positive-working photosensitive composition capable of forming a patterned resist layer having excellent stability in dimensions and suitable for use, for example, in fine working of various semiconductor devices.
As is well known, the manufacturing process of many semiconductor devices, such as transistors, ICs, LSIs and the like, involves a step of photoetching. In the photoetching, a uniform layer of a photoresist material is formed on the surface of a silicon wafer and the photoresist layer is subjected to pattern-wise exposure to light through a photomask having a desired pattern and kept in direct contact with the photoresist layer followed by development to form a pattern-wise photoresist layer and etching of the exposed surface of the substrate to which selective diffusion of a dopant is subsequently undertaken. Usually, the above described process of photolithography to selective diffusion is repeated several times followed by forming electrodes and wirings by deposition of aluminum to finish a semiconductor device.
In the above described procedure for the manufacture of a semiconductor device including several times of photolithographic processing and selective diffusion, it is an unavoidable consequence that certain differences in levels of 1 .mu.m or larger are produced on the surface. This difference in levels may be even larger after a passivation treatment.
When electrodes and wirings are to be formed on the wafer surface, a coating layer of aluminum is first formed thereon by the techniques of vacuum vapor deposition and the coating layer of aluminum must be patterned by the photoetching method. Namely, a photoresist layer is formed on the aluminum layer and exposed to light patternwise while the photoresist layer cannot be free from a strong effect of halation from the highly reflective aluminum surface to cause some problems. This problem due to halation is particularly serious when the vapor deposition of aluminum has been performed on the surface of a silicon wafer having a large difference in surface levels since the incident light beams projected perpendicularly to the surface of the wafer may be reflected not only at the flat areas of the surface but also at the boundary portion between areas having a difference in levels. Since the surface of the substrate at such a boundary portion is more or less inclined, the reflected light also contains an irregularly reflected component. Such an irregular reflection of light on the aluminum surface is one of the major reasons for the poor reproducibility of very fine line patterns having a line width of a few .mu.m or smaller.
Various attempts and proposals have been hitherto made to decrease the undesirable effect due to halation by the reflection on the aluminum surface. One of the methods developed so far is the admixture of the photoresist composition with a photoextinctive dye. For example, Japanese Patent Kokai 59-142538 discloses a class of photoextinctive dyes suitable for such a purpose including specific azo compounds having at least one hydroxy group in a molecule.
These photoextinctive dyes are indeed effective to greatly reduce the halation in the photoresist layer admixed therewith in comparison with the same photoresist layer without the dye. The effectiveness of these dyes, however, is limited and not sufficient to comply with or follow up the rapidly increasing requirements in recent electronic technology toward finer and finer patterning of the photoresist layer since any smallest halation may badly affect the quality of the pattern reproduction. On a substrate surface having differences in levels, moreover, the thickness of the coating layer of the photoresist composition unavoidably cannot be very uniform allover the whole surface since the thickness is larger on the area at the lower side of the boundary between different levels than at the higher side. When a photoresist layer having such a difference in the thickness of the photoresist layer is exposed to light, the exposure dose on the area of a smaller thickness of the photoresist layer may exceed the optimum dose even when the same dose on the areas having a larger thickness of the photoresist layer is optimum or smaller. Needless to say, an overly exposure to light is accompanied by the enhancement of the adverse effect of halation to cause a problem that the patterned photoresist layer after development is not a high-fidelity reproduction of the original pattern with a decreased line width of the patterned line or the patterned line in the photoresist layer may have a cross sectional form deviated from ideal rectangularity. Therefore, any smallest halation may be responsible for the poor dimensional stability of the patterned resist layer. In some cases, a reproduction of a line pattern on the photomask may be a patterned photoresist layer of a broken line. Thus, it is imperative to minimize the problem due to halation in order to satisfy the requirements for extremely fine patterning of the photoresist layer on an aluminum layer.